nasm x86 Assembly Quick Reference ("Cheat Sheet")


mov dest,src
Move data between registers, load immediate data into registers, move data between registers and memory.
mov eax,4  ; Load constant into eax
mov ebx,eax  ; Copy eax into ebx
mov ebx,[123] ;  Copy ebx to memory address 123
push src
Insert a value onto the stack.  Useful for passing arguments, saving registers, etc.
push ebp
pop dest
Remove topmost value from the stack.  Equivalent to "mov dest,[esp]     add esp,4"
pop ebp
call func
Push the address of the next instruction and start executing func.
call print_int
Pop the return program counter, and jump there.  Ends a subroutine.
add src,dest
add eax,ebx # Add ebx to eax
mul src
Multiply eax and src as unsigned integers, and put the result in eax.  High 32 bits of product go into eax.
mul ebx #Multiply eax by ebx
jmp label Goto the instruction label:.  Skips anything else in the way. jmp post_mem
cmp a,b

Compare two values.  Sets flags that are used by the conditional jumps (below). 
cmp eax,10 
jl label Goto label if previous comparison came out as less-than.  Other conditionals available are: jle (<=), je (==), jge (>=), jg (>), jne (!=), and many others. jl loop_start  ; Jump if eax<10

Stack Frame

(example without ebp or local variables)
off esp
caller's variables
Argument 2
Argument 1
Caller Return Address

my_sub: # Returns first argument
  mov eax,[esp+4]

(example when using ebp and two local variables)
off ebp
off esp
caller's variables
Argument 2
Argument 1
Caller Return Address
Saved ebp
Local variable 1
Local variable 2
[ebp-8] [esp]

my_sub2: # Returns first argument
  push ebp  # Prologue
  mov ebp, esp
  mov eax, [ebp+8]
  mov esp, ebp  # Epilogue
  pop ebp

Constants, Registers, Memory

"12" means decimal 12; "0xF0" is hex.  "some_function" is the address of the first instruction of a label.
Memory access (use register as pointer): "[esp]".  Same as C "*esp".
Memory access with offset (use register + offset as pointer): "[esp+4]".  Same as C "*(esp+4)".
Memory access with scaled index (register + another register * scale): "[eax + 4*ebx]".  Same as C "*(eax+ebx*4)".


esp is the stack pointer
ebp is the stack frame pointer
Return value in eax
Arguments are on the stack
Free for use (no save needed):
   eax, ebx, ecx, edx
Must be saved:
   esi, edi, ebp, esp
8 bit: ah (high 8 bits) and al (low 8 bits)
16 bit: ax
32 bit: eax
64 bit: rax

The Intel Software Developer's Manuals are incredibly long, boring, and complete--they give all the nitty-gritty details. Volume 1 lists the processor registers in Section 3.4.1. Volume 2 lists all the x86 instructions in Section 3.2.  Volume 3 gives the performance monitoring registers in Section. For Linux, the System V ABI gives the calling convention on page 39. Also see the Intel hall of fame for historical info. has a good opcode table.  Ralph Brown's Interrupt List is the definitive reference for all interrupt functions.  See just the BIOS interrupts for interrupt-time code.

O. Lawlor,
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